Many types of non-aqueous electrolyte batteries are known, and lithium ion secondary batteries can be given as a typical non-aqueous electrolyte battery. Lithium ion secondary batteries can be used at normal temperature, are high in both capacity and energy density, and have excellent cycle characteristics. For this reason, lithium ion secondary batteries are widely used as the power source for portable small-size electronic devices such as cellular phones, personal digital assistants (PDA), laptop computers, and video cameras. With achievements of high-level functionality and the like in compact electronic devices as mentioned above, the achievement of higher capacity in lithium ion secondary batteries is being anticipated.
In response to this demand, high-capacity positive electrode active materials and high-capacity negative electrode active materials are undergoing development anew. In particular, expectations are high for a lithium ion secondary battery using an alloy-type negative electrode active material which can be alloyed with lithium. Known examples of an alloy-type negative electrode active material are: a simple substance of silicon (Si) or tin (Sn); an oxide of either thereof; and an alloy containing Si or Sn. An alloy-type negative electrode active material has a high discharge capacity and is thus effective in achieving higher capacity in a lithium ion secondary battery. For example, the theoretical discharge capacity of silicon is about 4199 mAh/g, which is about 11 times higher than the theoretical discharge capacity of graphite.
In this manner, since a lithium ion secondary battery including an alloy-type negative electrode active material has a high energy density, its current value becomes higher compared to a conventional lithium ion secondary battery. Thus, the battery temperature tends to rise easily. Therefore, being anticipated are highly-safe lithium ion secondary batteries in which: in the case where an external short circuit occurs, discharge is not conducted until the possibility arises of the battery temperature rising rapidly; and discharge can be reliably stopped.
On the other hand, Patent Document 1 proposes adjusting the total cross-sectional area of the plurality of leads connected to the positive electrode current collector and the negative electrode current collector in a lithium ion secondary battery, the adjustment made in accordance with the material of the leads. In Patent Document 1, amorphous carbon materials such as soft carbon and hard carbon, or carbon powders such as of artificial graphite and of natural graphite are used as the negative electrode active material, while alloy-type negative electrode active materials having a higher capacity are not. In addition, the total cross-sectional area of the leads is adjusted in Patent Document 1 in order to prevent the leads from melting, even in the case where a large current of at least 100 A flows. By the above, an attempt is made to achieve the goal of making the battery operate reliably at all times. This is due to the fact that the lithium ion secondary battery of Patent Document 1 is for use as the power source for motor drives in hybrid electric cars and the like.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 11-345630